While many sci-fi writers have wondered if time travel into the past can be allowed as long as you slip into an alternate universe, you should be warned: Nature doesn’t like a cheater.
Time travel into the past is a real pain in the neck. For one, as far as we can tell it’s forbidden in our universe. You can travel into the future as slowly or as quickly as you like, but you can’t stop time or go in reverse.
Technically speaking, physicists are not exactly sure why time travel is forbidden. We don’t have a single law of physics that clearly rejects the possibility.
But every time we try to concoct a time travel concept — like wormholes or twisting paths around infinitely long cylinders — we find some reason that it’s not allowed. Wormholes, it turns out, require negative mass to stabilize themselves, and infinitely long cylinders ...
